1. Field of the Invention
The present invention relates generally to solar receivers capable of heating a recirculating airstream, and more particularly to methods and systems for inhibiting ambient heat losses in such receivers resulting from wind interference and the like.
Central solar receivers produce energy directly from the sun and promise to provide stationary power sources which are substantially free from all forms of pollution. While a number of specific designs have been proposed for constructing central solar receivers, they generally rely on placing an elevated receiver assembly at a focus point located in a large field of heliostats (mirrors). The heliostats reflect incident sunlight onto the receiver where a heat exchange unit is provided to raise the temperature of a circulating heat exchange medium. Heat may then be extracted from the heat exchange medium and used, for example, to generate steam which can then be used for conventional electrical generation.
Of particular interest to the present invention, it has been proposed to use central solar receivers which employ an air permeable heat exchange unit which is capable of raising the temperature of a circulating airstream which passes through the unit and acts as the heat exchange medium. To enhance efficiency and reduce capital costs, the heat exchange member is exposed directly to the ambient with the recirculating airstream being introduced through the exposed side and being collected on an enclosed side of the receiver. As might be expected, however, ambient winds can interfere greatly with the flow of air into the heat exchange element and provisions must be made to reduce such interference.
To overcome such wind losses, it has been proposed to recess the heat exchange element well inside an enclosure and to direct an open aperture in the enclosure downward so that the warm air tends to remain entrapped inside the enclosure, thus reducing mixing and ambient losses. Such an approach is illustrated in U.S. Pat. No. 4,312,324. While generally workable, the approach suffers from certain drawbacks. The necessity of orienting the aperture downward requires construction of a taller and more expensive tower, or alternatively, limits area over which the heliostat field may be constructed which in turn limits the amount of solar energy that can be delivered to the receiver. Moreover, the need to recess the heat exchange element increases the capital cost of constructing the solar receiver as it requires substantial additional structure.
U.S. Pat. No. 4,312,324, further teaches the provision of a concentrator and passive deflector circumscribing the open aperture in the solar receiver enclosure. The patent also teaches that active wind deflection may be provided by the use of a plurality of air nozzles projecting substantially away from the open aperture. While these approaches may provide limited protection from ambient winds, they both rely on the receiving element being recessed well within the receiving enclosure. Moreover, they rely on the aperture being sized to have a cross-sectional area much less than that of the heat exchange element. Neither would be useful in constructions where the heat exchange element is exposed directly to the ambient and not recessed behind a reduced-diameter aperture.
For these reasons, it would be desirable to provide solar receivers having heat exchange elements which are directly exposed to the ambient and which do not need to be recessed or inclined downward in order to inhibit heat losses. In particular, it would be desirable to provide systems and methods for protecting such solar receivers from heat losses resulting from ambient winds.
2. Description of the Background Art
U.S. Pat. No. 4,312,324 is discussed hereinabove. U.S. Pat. No. 4,777,934 describes a windowless solar receiver which employs a recirculating particulate-laden airstream for absorbing reflected solar radiation. An air curtain flowing generally parallel to the particulate-laden airstream protects the particulate-laden airstream from ambient disturbances. Central solar receivers intended for heating air on a once-through basis are generally described in U.S. Pat. Nos. 4,777,935; 4,676,068; 4,683,872; and 4,394,859. U.S. Pat. No. 3,875,925, describes a porous barrier which is heated by the sun and transfers thermal energy to air passing therethrough. The use of air curtains to protect windowless solar receivers is discussed generally in Final Reports prepared under Department of Energy Contract No. DE-AC03-83SF11693 and No. DE-AC03-83SF11940.